I just received a document from Pennsylvania Department of Revenue claiming that I owe them $366 for 2018 tax year and I don't know why. I can send a copy of the document

[KarenJ]

There should be a phone number on the form you received.  The best way to determine why they assessed an additional tax would be to call them at that number.

If there is no phone number on the form, you can go onto their website where they have information regarding tax notices.   https://revenue-pa.custhelp.com/app/answers/list/c/228,229

Or you can call their phone number 717-787-8201